Rapid development of contrast agents in the recent years has generated a number of different compositions and formulations, which are useful in contrast-enhanced imaging of organs and tissues of human or animal body as well as in therapeutic treatments thereof.
A class of contrast agents particularly useful for Contrast Enhanced UltraSound imaging (“CEUS” imaging) includes suspensions of gas bubbles of nano- and/or micro-metric size dispersed in an aqueous medium. The gas is typically entrapped or encapsulated in a film-layer comprising, for instance, emulsifiers, oils, thickeners or sugars. These stabilized gas bubbles (dispersed in a suitable physiological solution) are generally referred to in the art with various terminologies, depending typically from the stabilizing material employed for their preparation; these terms include, for instance, “microspheres”, “microbubbles”, “microcapsules” or “microballoons”, globally referred to here as “gas-filled microvesicles” (or “microvesicles”).
UltraSound Contrast Agents (“USCA”s) are manufactured according to various manufacturing methods. One of these methods, see e.g. WO 94/09829(1), entails the dissolution of a mixture of film-forming components (such as phospholipids and/or fatty acids) and of a hydrophilic stabilizing compound (e.g. polyetheleneglycol) in an organic solvent; the obtained mixture is thus filled into vials which are subjected to freeze-drying (lyophilization). The vials containing a solid freeze-dried solid residue (“cake”) at the bottom thereof are then filled with a suitable gas (e.g. a fluorinated gas) and finally sealed for storage. Before use, an aqueous suspension of microbubbles is easily prepared by injecting a suitable liquid into the vial (e.g. saline) and shaking the vial to dissolve the solid residue.
A commercially available USCA which can be manufactured according to the above method is SonoVue® (or Lumason® in the USA), from Bracco.
The Applicant has now observed that the polyethylene glycol employed in the preparation of the lyophilized “cake” may have variations in its characteristics which may negatively affect the number of gas-filled microvesicles obtained upon reconstitution of the lyophilized powder.
In particular, the Applicant has observed that different releases (even from a same manufacturer) of commercially available PEG4000 may have different amounts of folded polymeric chains in the polymeric material. As observed by the Applicant, if the percentage of folded chains in the polymeric material is too low, this may result in a too high number of vials falling to pass the acceptability test in a manufacturing batch. As under an industrial scale a manufacturing batch may comprise few thousand of vials, it may well be understood that even a relatively low number of discharged vials is highly undesirable.
Based on the above observation, the Applicant has determined that the polyethylene glycol used in the formulation of the lyophilized powder for the preparation of gas-filled microvesicles shall have a percentage of folded polymeric chains higher than a predetermined value.